Low voltage electric cables (e.g. up to 1 kV) such as those used in underground installations are generally made of conductor strands covered by an insulating layer. A puncture in the insulating layer of a cable may happen during the cable installation, during prior manipulations such as when winding the cable around a drum, or even as a result of a manufacturing defect. While the cable is energized, in presence of humidity or water, an electric arc or a partial discharge can occur between the conductor of a damaged cable and the neutral, causing degradation of the insulating layer and an eventual failure condition. It can even lead to an explosive environment if the cable is installed in a duct and an explosive gas is produced as a result of the insulation decomposition. This phenomenon is discussed in the article “Manhole Explosions Due to Arcing Faults on Underground Secondary Distribution Cables in Ducts”, Bohdan, K. et al., IEEE Transactions on Power Delivery, Vol. 7, No. 3, July 1992, p. 1425-1433, and in the article “Intermittent Arcing Fault on Underground Low-Voltage Cables”, Hamel, A. and al., IEEE Transactions on Power Delivery, Vol. 19, No. 4, October 2004, pp. 1862-1868.
There is thus a need for testing the integrity of the insulating layer of a cable, whether it is a new cable received around a drum, a cable just installed, or an old cable in service, and in particular in the case of an underground electric cable.
As discussed in U.S. Pat. No. 6,600,423 (Rozier et al.), telecommunication companies use air pressure pumped into their cables and pipes to create positive pressure in the cables, enabling them to resist standing water, moisture damage and the like.
U.S. Pat. No. 5,596,138 (Onodera et al.) discusses pressure testing an electrical housing such as that of an electric plug for air tightness.
U.S. Pat. No. 5,238,172 (Le Davay) discusses sealing integrity testing of a metal tube that encases an electrical and/or optical fiber cable embedded in a sealing material filling the tube. The testing relies upon priming the sealing material injected into the tube with a detectable test gas.
The above techniques do not address the problem of testing the integrity of an insulating layer around a cable having a longitudinal void such as formed, for example, by the interstitial space along a conductor strand or the like, in order to detect damage such as caused by a puncture and to avoid cable failure and potential safety hazards.
For a cable in a dry pipe, a dielectric test would not detect a small insulation defect.
Self sealing cable technology that contains a migrating fluid in the entire cable with the fluid being inside the insulation layers is also known. However, the self sealing technology is required to be used throughout the whole length of the cable to provide effective repair protection, whether the cable has been damaged or not, which is not an economical solution.